In recent years, with the redundant arrays of inexpensive disks (RAID) technology, a disk array which realizes high-speed data access and high capacity by redundantly arranging a plurality of storage devices (e.g., hard disk drive (HDD) or solid state drive (SSD)) is widely used. In the meantime, as the number of storage devices increases in order to increase the capacity of the disk array, the amount of power consumed by the disk array increases. As a result, from the standpoint of reducing power costs and environmental loads, researches and developments of power saving in the disk array have been in progress.
As a technique for achieving power consumption reduction of the disk array, there is so called “massive array of idle disk (MAID)”. The MAID is a technology for reducing the power consumption by causing a storage device in which data access has not occurred in the disk array to shift to a power saving mode (e.g., reduction of a disk rotational speed, stop of the disk rotation, retraction of a disk head, power-off, or the like). In the disk array to which the MAID is applied, since the storage device with data access is operated normally, an influence on the high speed performance and reliability is low.
However, when data access occurs in a storage device in the power saving mode, it takes time for the storage device to return to a normal operation mode, and the data access is delayed. In order to suppress the delay, there has been proposed a technique for estimating a probability of data access to the data to be written, and determining a storage device as a writing destination on the basis of the estimated probability. Further, a distributed storage system has been proposed, which includes a storage device having a disk array and a cache server for caching data with concentrated data access.
Related techniques are disclosed in, for example, International Publication Pamphlet No. WO 2011/121869 and Japanese Laid-open Patent Publication No. 2010-002942.
If a storage device storing a large amount of data with low probability of data access is shifted to a power saving mode, it is possible to reduce a risk such as a decrease in a high speed performance due to a data access occurring in the storage device in the power saving mode. However, an effect of the risk reduction is not expected if accuracy of the estimation is low. If data with high access frequency is saved in a cache server, since the cache server may respond to a case where access to the saved data occurs, it is possible to reduce a risk such as a decrease in high speed performance of data access.
However, since there is a limit to the storage capacity of the cache server, it is practically difficult to cache the data with high access frequency in all the storage devices. When storage devices in which all the data with high access frequency has been cached is shifted to the power saving mode and the other storage devices are operated in the normal operation mode, the high speed performance of data access is maintained. However, the number of storage devices shifted to the power saving mode is limited. As a result, the effect of reducing the power consumption becomes limited.